1. Field of the Invention
The present invention relates to a semiconductor device and method for manufacturing the semiconductor device, and more particularly to an insulating film used in the semiconductor device.
2. Description of the Related Art
In accordance with the development of MOSFET microfabrication, there is a demand for thinning a gate dielectrics (gate insulating film). A conventional silicon oxide film or silicon oxynitride film cannot have a thickness less than about 2 nm because of an increased direct tunnel current.
In light of this, it has been proposed to use, as a gate dielectrics, a silicon oxide film containing a metal (i.e., a metal silicate film, also called a “silicate film”). Since the metal silicate film has a higher dielectric constant than a silicon oxide film, and a relatively high crystallization temperature, it is suitable for a polysilicon (or poly-SiGe) gate electrode forming process.
Further, to suppress diffusion of boron from a gate electrode, the use of an oxynitride metal silicon film has been proposed (see Jpn. Pat. Appln. KOKAI Publication No. 2000-49349). However, an oxynitride metal silicon film having good interface characteristics has not yet been developed. Moreover, since a metal nitride has a conductive property, a large amount of leakage current flows in the film. Further, the film has a high trap density. In addition, a metal silicide may be formed at a gate electrode interface, thereby degrading the insulation property of the film.
A CVD (Chemical Vapor Deposition) method using an organic silane is given as an example of a metal-silicate-forming method. Among organic silanes, tetraethoxysilane (Si(OC2H5)4: TEOS) is often used. Since TEOS has a high decomposition temperature, a temperature not less than 700° C. is necessary if a thermal CVD method is used. To execute a film-forming process at a lower temperature, ozone (O3) may be used together with TEOS, or a plasma CVD method may be used.
When using ozone or plasma, however, active oxygen species such as oxygen radicals or oxygen ions will be inevitably created in the film-forming atmosphere. Active oxidation species have a high reactivity and hence oxidize the underlayer of the device. Further, in the case of using plasma, the underlayer may be damaged by plasma.
The following publications disclose various methods for forming a metal silicate film.
Jpn. Pat. Appln. KOKAI Publication No. 5-239650 discloses a process for adding alkoxide of a titanium group element or an alkylamine compound in a CVD process using alkoxy silane as a source. This process uses ozone or plasma, and is therefore considered a process that uses active oxygen species.
Jpn. Pat. Appln. KOKAI Publication No. 6-160657 also discloses a process using ozone, i.e., an active oxygen species.
Jpn. Pat. Appln. KOKAI Publication No. 11-111715 discloses a process for adding, to a source gas, a product resulting from thermal decomposition of a compound containing alkoxide. However, there is no description concerning mixing of a silicon source and a metal source.
Jpn. Pat. Appln. KOKAI Publication No. 5-226608 discloses the use of titanium as a metal contained in a metal silicate film. It is difficult, however, to produce satisfactory characteristics from a metal silicate film containing titanium, and hence to use the film in a semiconductor device.
As described above, in the prior art, it is difficult to obtain a metal silicate film having excellent characteristics, and/or the underlayer may be adversely affected by forming the film. This means that it is difficult to obtain a semiconductor device having excellent characteristics and high reliability.